Metal halide arc discharge lamps are widely used in commercial and industrial applications. A generally cylindrical arc tube contains a fill material including a starting gas, mercury and one or more metal halides. An electrode is located at each end of the arc tube. The arc tube is located within an outer envelope having a base for mounting the lamp and for connection to an electrical power source.
Metal halide lamps are sensitive to orientation. As a result, such lamps are usually designed either for vertical or horizontal operation. A metal halide lamp designed for vertical operation is disclosed in U.S. Pat. No. 4,812,714 issued Mar. 14, 1989 to Keeffe et al. A metal halide lamp designed for horizontal orientation is disclosed in U.S. Pat. No. 4,687,453 issued Aug. 18, 1987 to Lekebusch et al. In horizontal arc lamps, the arc arches upwardly due to an interaction between the arc and gravity. In practice, it is difficult to properly align these lamps.
When metal halide arc lamps, particularly those of 200 watts or greater, are operated at orientations other than the orientation for which they were designed, the operating life is typically reduced. Thus, it has been customary to take precautions to ensure that the lamp is installed with the proper orientation. It is sometimes inconvenient to operate a metal halide lamp in the orientation for which it was designed. In other cases, the lamp is inadvertently installed with the improper orientation, and the lamp life is reduced. It is thus desirable to provide, a metal halide arc lamp which is not sensitive to orientation.
Discharge lamps having more than two electrodes are known in the art. Metal vapor discharge lamps having three or more electrodes for operation from polyphase power are disclosed in U.S. Pat. No 2,265,323 issued Dec. 9, 1941 to Spanner and in British Patent No. 643,321 published Sept. 20, 1950. Operation from polyphase power is reported to reduce lamp flicker. A polyphase high pressure gas discharge lamp having a spherical lamp envelope and three electrodes is disclosed in Japanese Patent No. 64-86442 published Mar. 31, 1989. A discharge lamp having 2n+1 electrodes to provide 2n discharge passages is disclosed in French Patent No. 959,659 published Apr. 3, 1950.
It is a general object of the present invention to provide improved arc discharge lamps.
It is another object of the present invention to provide arc discharge lamps which can be operated in any orientation in a gravity field.
It is yet another object of the present invention to provide arc discharge lamps which have a long operating life.
It is a further object of the present invention to provide arc discharge lamps which can be operated at different power levels.
It is a further object of the present invention to provide an arc discharge lamp wherein operating power is automatically applied to a selected pair of electrodes.
It is yet another object of the present invention to provide an arc discharge lamp having an electrode configuration which permits operation in any orientation without substantial reduction in the operating life of the lamp.
It is still another object of the present invention to provide arc discharge lamps which are easily installed.